The present invention refers to a method of manufacturing fuselage barrels in composite material stiffened by stringers. More particularly, the invention refers to the manufacturing of fuselage barrels made in a single piece in composite material, including a skin of overall tubular, cylindrical or truncated conical shape, and a series of longitudinal stringers which protrude on the inner surface of the barrel.
The state of the art includes various methods for manufacturing complete fuselage barrels in composite. In general, some methods require curing the barrel around an inner mandrel or inner forming tool, which provides the inner forming surface for the barrel (the so-called “inner mould line” (IML) or “inner mould surface”). In other cases, curing takes place in a hollow forming tool which provides the outer forming surface for the barrel (“outer mould line” (OML) or “outer mould surface”).
A first known method includes laminating the barrel and its subsequent curing over a mandrel having a shape corresponding to the inner surface (“inner mould line”) of the fuselage barrel. According to this technique, the not yet cured stringers are accommodated in corresponding longitudinal slots formed in the outer surface of the mandrel. An inflatable element (or “bladder”) is positioned inside each stringer, and then a layer of fabric pre-impregnated with thermosetting curable resin is laid over the stringers in order to retain the stringers and the inflatable elements during the subsequent rotation stage. The outer skin is then laid down using the known art of fibre placement. Flexible sheets of metal or carbon (so-called “caul plates”) are then positioned over the assembly, their function being to smooth the outer surface of the barrel during curing in the autoclave. Finally, the entire assembly is covered by a vacuum bag made by a known technique using a nylon film, after preparing pathways for extracting the air and the gases (breather fabric and vacuum valves). The whole assembly is then positioned in an autoclave for the process of curing/compaction by means of heat and temperature.
This method has the advantage of ensuring a precise inner surface which simplifies the assembly of the inner structural elements (spar frames etc.) but potentially entails some drawbacks:                In order that it can be extracted from the barrel after curing, the mandrel, which has the shape of the IML, is dismantlable into a multiplicity of collapsible sectors (typically six sectors). This requirement implies the presence of sealing gaskets between the sectors which, as they have to resist the temperature and pressure of the autoclave, must be maintained frequently to avoid the risk of sealing losses with consequent unacceptable porosity on the barrel produced.        Breaking down the mandrel into radially retractable sectors requires operating mechanisms. The latter are negatively affected by the temperature and pressure of the autoclave, and for this reason they, too, are subject to frequent maintenance operations.        The sectors of the mandrel must be accurately coupled in order to prevent the formation of unacceptable steps and wrinkles on the barrel. For large-sized fuselage barrels, accurate coupling of the sectors is very complex and requires frequent adjustments.        A compaction mechanism thrusts the outer surface of the barrel against the IML surface of the shaping mandrel, reducing the circumferential length of the uncured material to bring it to the cured thickness. The action of the compaction mechanism, combined with the reduction in the circumferential length, creates longitudinal wrinkles on the outer surface of the barrel which must be characterized by a complex and expensive plan of structural tests to verify acceptability.        
Another known method is used to produce fuselage barrels of small diameter, with a sandwich structure and a honeycomb stiffening. In this method, an auxiliary mandrel is used, of a shape corresponding to the inner shape of the barrel (IML), on which a nylon film is arranged which will constitute the final vacuum bag. The inner skin is then laid down by means of the known art of fibre placement. Subsequently, the said “honeycomb” material is positioned and the outer skin of carbon-reinforced resin is then laid down, again using fibre placement technique. At this point a curing tool with a surface equal to the outer surface of the barrel (outer mould line, OML) is positioned around the barrel. The vacuum bag, already set up on the IML auxiliary mandrel, is sealed including to the OML outer cure tool. When the vacuum is applied, the barrel tends to separate from the IML auxiliary mandrel to attach itself to the surface of the OML mandrel. A gap is thus created which makes it possible to remove the IML auxiliary mandrel and to send the barrel in the OML tool into the autoclave for curing/compaction. This method overcomes the problems of the previous method, but is applicable with known methods only to sandwich structures, while the risk remains that extracting the IML mandrel from the barrel before curing, which is performed with a limited gap, could damage the curing bag.
The need is currently felt in the aeronautical industry for manufacturing barrels stiffened by stringers.
US 2009/0139641 A1 provides for the application of the composite material to the inside of a hollow surface, constituting the OML surface, of a hollow mandrel, so as to form a skin which extends for 360° around an axis. The stringers are then positioned on an inner surface of the skin, by means of a positioner which enters the inside of the mandrel OML and thrusts the flanges or wings of the stringers against the skin. A vacuum bag is installed over the stringers and a vacuum is applied to it in order to press the stringers and the skin outwards and against the inner surface of the tool. The assembly of skin and stringers is then co-cured to fix the stringers to the skin and harden the structure. For laminating, a special fibre placement machine is required, capable of laminating inside the OML mandrel, as well as a complex apparatus for positioning the stiffening stringers and the vacuum bag.